Ethanoic acid is a carboxylic acid used in vinegar. A student adds a small piece of calcium carbonate to a sample of ethanoic acid solution. Describe what the student would observe and explain what is happening, including the products formed in the reaction.

OCR A-Level Chemistry B: Salters (H433) — 6.1 Aromatic compounds, carbonyls and acids · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Ethanoic acid is the second member of the homologous series of carboxylic acids. Its functional group is –COOH. Carboxylic acids have typical acidic properties, including reactions with carbonates.

Model answer (5 marks)

Bubbles of CO₂ are seen as the mixture fizzes.
The solid calcium carbonate disappears as it reacts.
The –COOH group donates a proton to the carbonate ion.
CO₂ gas is released, giving the effervescence.
The products are calcium ethanoate (Ca(CH₃COO)₂) and water.

Examiner tips

  • Use the word ‘fizz’ or ‘effervescence’ to show you know the visual cue. Mention the proton transfer from –COOH to carbonate. State the products explicitly: Ca(CH₃COO)₂ and H₂O.
  • common_mistakes
  • :
  • Saying only CO₂ is produced and ignoring the calcium salt. Forgetting to mention the disappearance of the solid. Using ‘acid’ instead of the specific –COOH group.”

Mark scheme (5 marks)

  1. Bubbles / effervescence are observed
  2. The calcium carbonate (solid) disappears / dissolves
  3. Carboxylic acids react with carbonates because they have typical acidic properties
  4. Carbon dioxide gas is produced (causing the bubbles)
  5. A calcium salt (calcium ethanoate) and water are also produced

Key terms in this question

carbonate

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